The Philadelphia Experiment is an alleged military experiment that is said to have been carried out by the U.S. Navy at the Philadelphia Naval Shipyard in Philadelphia, Pennsylvania some time around October 28, 1943. The U.S. Navy destroyer escort USS Eldridge was claimed to be rendered invisible (or "cloaked") to enemy devices.

The story is thought to be a hoax. The U.S. Navy maintains that no such experiment was ever conducted, and details of the story contradict well-established facts about the Eldridge itself, as well as commonly accepted physics.